1. Field of the Disclosure
This disclosure relates generally to the acquisition of seismic data using seismic systems and methods that employ wireless communication between field data acquisition units and one or more remote units.
2. Background of the Art
Seismic surveys are conducted to map subsurface structures to identify and develop oil and gas reservoirs. Seismic surveys are typically performed to estimate the location and quantities of oil and gas fields prior to developing (drilling wells) the fields and also to determine the changes in the reservoir over time subsequent to the drilling of wells. On land, seismic surveys are conducted by deploying an array of seismic sensors (also referred to as seismic receivers) over selected geographical regions. These arrays typically cover 75-125 square kilometers or more of a geographic area and include 2000 to 5000 seismic sensors. The seismic sensors (geophones or accelerometers) are placed are coupled to the ground in the form of a grid. An energy source, such as an explosive charge (buried dynamite for example) or a mobile vibratory source, is used at selected spaced apart locations in the geographical area to generate or induce acoustic waves or signals (also referred to as acoustic energy) into the subsurface. The acoustic waves generated into the subsurface reflect back to the surface from subsurface formation discontinuities, such as those formed by oil and gas reservoirs. The reflections are sensed or detected at the surface by the seismic sensors. Data acquisition units (also referred to herein as the field service units or “FSUs”) deployed in the field proximate the seismic sensors receive signals from their associated seismic sensors, at least partially processes the received signals, and transmit the processed signals to a remote unit (typically a central control or computer unit placed on a mobile unit). The central unit typically controls at least some of the operations of the FSUs processes the seismic data received from all of the FSUs, records the processed data on data storage devices for further processing. The sensing, processing and recording of the seismic waves is referred to as seismic data acquisition.
Two-dimensional and/or three-dimensional maps of the subsurface structures (also referred to as the “seismic image”) are generated from the seismic data recorded by the central. These maps are then used to make decisions about drilling locations, reservoir size, pay zone depth and estimates of the production of hydrocarbons.
The traditional sensor used for acquiring seismic data is a geophone. Multi-component (three-axis) accelerometers, however, are more commonly used for obtaining three-dimensional seismic maps compared to the single component sensors seismic surveying layouts using multi-component sensors require use of more complex data acquisition and recording equipment in the field and a substantially greater bandwidth for the transmission of data to a central location.
A common architecture of seismic data acquisition systems is a point-to-point cable connection of all of the seismic sensors. Typically, output signals from the sensors in the array are collected by data acquisition units attached to one or more sensors, digitized and relayed down the cable lines to a high-speed backbone field processing device or field box. The high-speed backbone is typically connected via a point-to-point relay fashion with other field boxes to a central recording system, where all of the data are recorded onto a storage medium, such as a magnetic tape.
Seismic data may be recorded at the field boxes for later retrieval, and in some cases a leading field box is used to communicate command and control information with the central recording system over a radio link (radio frequency link or an “RF” link). Even with the use of such an RF link, kilometers of cabling among the sensors and the various field boxes may be required. Such a cable-system architecture can result in more than 150 kilometers of cable deployed over the survey area. The deployment of several kilometers of cable over varying terrain requires significant equipment and labor, often in environmentally sensitive areas.
FIG. 1 (prior art) depicts a conventional cable seismic data acquisition system 100. Such a system includes an array (string) of spaced-apart seismic sensor units 102. Each string of sensors is typically coupled via cabling to a data acquisition device 103, and several of the data acquisition devices and associated string of sensors are coupled via cabling 110 to form a line 108, which is then coupled via cabling 112 to a line tap or (crossline unit) 104. Several crossline units 104 and associated lines are usually coupled together by cabling, such as shown by the dotted line 114.
The sensors 102 are usually spaced between 10-50 meters. Each of the crossline units 104 typically performs some signal processing and then stores the processed signals as seismic information. The crossline units 104 are each typically coupled, either in parallel or in series, with one of the units 104a serving as an interface between the central controller or control unit (CU) 106 and all crossline units 104. In the cable system of FIG. 1, data are usually relayed from one sensor unit to the next sensor unit and through several field boxes before such data reaches the central controller. Failure of any one field box or cable can cause loss of recording of large amounts of information. Operators often halt the surveying activity until the source of the problem is determined and corrected. Consequently, common cable systems can have a relatively low average uptime, often only 45%.
The basic architecture and reliability issues of the current cable systems described above prevent seismic data acquisition systems from being scaled to significantly higher channel counts. Some cable systems incorporate different levels of redundancy to address the issue of single-point failure. These redundant systems can include multiple redundant backbones, telemetry reversal and other redundancy features. These solutions, however, require even more cable to be deployed on the ground and still limit fault tolerance to a few, often no more than two failures, in a line that can be many miles long.
Optimal spacing between seismic sensors varies depending on desired image depth and type. Obstacles are often encountered when deploying sensors such as no permit areas, rivers, and roads that cause the seismic crew to use varying spacing between sensor stations. Varying the distance between sensors in a conventional cable system is not convenient due to the fixed interval between connection points. Usually a surveying crew predetermines the appropriate positions of the sensors on the ground prior to laying out the acquisition equipment. A global positioning system (GPS) receiver is then used by the surveyor to plant stakes in the ground at each of the thousands of predetermined sensor locations. Therefore, array deployment in such systems is a two-step process, which increases the time and labor costs of the seismic survey process.
Wireless seismic data acquisition systems have been proposed to address many of the problems associated with the cable seismic data acquisition systems. In the cable systems, large amounts of data can be transmitted over the cable connections, including problems detected by the field boxes or specific data requested or polled by the CU from the various field boxes. The wireless systems utilize radio frequency transmission and are typically bandwidth limited. In traditional wireless seismic data acquisition systems, an attribute (physical or seismic) degradation affecting the data quality is typically detected by monitoring (printing and viewing) shot (source activation) records immediately after recording. However, with ever-increasing channel counts on three dimensional seismic surveys, the bandwidth necessary for transmitting each record in real-time can be difficult.
To preserve bandwidth and to reduce or eliminate the need to monitor individual records for quality control, it is desirable to have a system in which the field devices can detect and appropriately or selectively transmit attribute degradation information (also referred to herein “alarm conditions”). Because several FSUs may detect one or more attribute degradation or other conditions simultaneously, it is desirable to manage the transmission of messages containing such information to the CU.
The present disclosure provides seismic surveying systems, apparatus and methods for managing the detection, collection and transmission of data, including messages relating to attribute degradation and other surveying conditions between the field units and a remote unit, such as a CU, central system computer (CSC) and/or an intermediate (repeater) unit, that address some of the above-noted shortcomings.